The peritoneum is either secondarily affected by carcinoma, a cancerous growth originally generated externally to it, ap-proaching and involving it in its metamorphosis, perforating it and penetrating into its cavity; or the carcinoma is produced without such antecedents, though commonly occasioned by a carcinomatous affection in the neighborhood, in the vicinity of which it is formed; or, lastly, it occurs in some rare cases, altogether independently of such causes, as a primary affection of the peritoneum. It must, therefore, with the exception of the last-mentioned unfrequent case, be considered as the product of cancerous cachexia which has already found a nidus and a local habitation.

The most common form of carcinoma, into which, however, the other carcinomatous growths, which give rise to its appearance on the peritoneum, usually degenerate, is the areolar, and, second in order, the medullary species.

The former appears as a hard, crystalline, transparent, and discrete cancerous follicle, resembling tubercle, and of the size of a hemp or millet seed; in the acute variety it is generally thickly sown over a large extent, and even spreads over the entire peritoneum, or it occurs as a layer of areolar cancerous tissue, varying in thickness, or as a circumscribed, round, lobulated aggregation. The omentum is very commonly found to shrivel up and to degenerate into a transverse band, or, in the opposite case, with an enormous increase of size, into areolar cancer.

Medullary carcinoma frequently occurs in the acute form, as the white or colored (melanosis) encephaloid variety, either deposited in layers, or more commonly as compressed, rounded, medullary nodules, of different dimensions.

In the fibro-carcinomatous degeneration of the peritoneum and its subserous cellular tissue, which occasionally extends over the entire peritoneum, we invariably perceive an atrophy and condensation of the tissues, and, in consequence, a contraction of the carcinomatous folds of the peritoneum, e. g. in the mesentery.

Peritoneal cancer is commonly complicated, in the manner above described, with gastric intestinal and ovarian cancer, and then also with uterine and hepatic cancer, the medullary form prevailing in the latter case. We must not omit to allude, at this place, to the nodulated retro-peritoneal cancerous formations of Lobstein, which commonly take their origin in the glands of the lumbar plexus or other subordinate portions of the absorbent system, and which extend into the mesentery.